The present invention relates to the manufacture of optical fibers.
Optical fiber used in communication systems typically includes a core of glass surrounded by a cladding also formed from glass having different optical properties from the core. The fiber typically is covered with a protective outer coating. Such fibers can be made by drawing a thin strand from a heated, partially molten preform formed from glass having the correct composition to make the core surrounded by a layer of glass having the appropriate composition to make the cladding. As a strand of soft, molten glass is pulled from the preform, both the core glass and the cladding glass stretch. The core remains in the middle and the cladding remains on the outside, thus forming the composite core and cladding structure of the finished fiber. As the fiber is pulled away from the preform, it cools and solidifies, and the coating is applied. These processes are performed at high speeds so that the fiber is drawn at high rates.
In operation of an optical communication system, light applied at one end of the fiber is pulsed or progressively varied in accordance with the information to be transmitted. The pulses or progressively varying light are received at the other end of the fiber. The speed at which light passes along a fiber depends upon many factors including the optical properties of the materials making up the core and cladding and, the diameter of the core. The fibers commonly used for optical data transmission systems are so-called xe2x80x9csingle modexe2x80x9d fibers. In these fibers, the core diameter is small enough that all of the light must pass through the core in a so-called xe2x80x9cfundamentalxe2x80x9d or xe2x80x9cHE11xe2x80x9d mode of transmission. Full discussion of transmission modes in optical fibers is beyond the scope of this disclosure. However, the fundamental or HE11 mode can be regarded as propagation of light straight along the axis of the core, as opposed to higher-ordered modes which can be thought of as propagation of light in a zig-zag pattern.
In a theoretically perfect single mode fiber, because all of the light passes through the fiber in the same mode, all light of a given wavelength will pass along the length of the fiber with the same velocity. However, the light passing along the fiber typically includes portions having different polarizations, i.e., different orientation of the electromagnetic waves constituting the light. If the fiber core is not perfectly cylindrical, but instead is out of round so that it has long and short diameters, light of one polarization will have its electrical waves aligned with a long diameter of the core whereas light of the other polarization will have its electrical waves aligned with the short diameter of the core. In this case, the effective diameter of the fiber core will be different for light of one polarization than for light of another polarization. Portions of light having different polarizations will travel at different velocities. Stated another way, the fiber has a xe2x80x9cslowxe2x80x9d axis in one direction perpendicular to its length, and a xe2x80x9cfastxe2x80x9d axis in the other direction perpendicular to its length.
Light having a direction of polarization aligned with the fast axis travels more rapidly than light having a direction of polarization aligned with the slow axis. As a result, the two polarization modes propagate with different propagation constants (xcex21 and xcex22). The difference between the propagation constants is termed birefringence (xcex94xcex2), the magnitude of the birefringence being given by the difference in the propagation constants of the two orthogonal modes:
xcex94xcex2=xcex21xe2x88x92xcex22. 
Birefringence causes the polarization state of light propagating in the fiber to evolve periodically along the length of the fiber. The distance required for the polarization to return to its original state is the fiber beat length (Lb), which is inversely proportional to the fiber birefringence. In particular, the beat length Lb is given by:
Lb=2xcfx80/xcex94xcex2
Accordingly, fibers with more birefringence have shorter beat lengths and vice versa. Typical beat lengths observed in practice range from as short as 2-3 millimeters (a high birefringence fiber) to as long as 10-50 meters (a low birefringence fiber).
In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence means that the two polarization modes travel at different group velocities, the difference increasing as the birefringence increases. The differential time delay between the two polarization modes is called polarization mode dispersion, or PMD. PMD causes signal distortion which is very harmful for high bit rate systems and analog communication systems.
This phenomenon is referred to in the art of fiber optic communication as polarization mode dispersion or xe2x80x9cPMDxe2x80x9d. Imperfections in the fiber other than differences in core diameter can also contribute to PMD. PMD causes distortion of the light pulses or waves transmitted along the fiber, thus reducing the signal quality and limiting the rate at which information can be passed along the fiber.
One method of reducing the effects of PMD is to continually re-orient the fast and slow axis of the fiber. This can be accomplished by spinning the fiber as it is drawn, so that the slow axis and the fast axis of the fiber are repeatedly interchanged along the length of the fiber. Thus, at one point along the length of the fiber the slow axis points in a first direction perpendicular to the length of the fiber and the fast axis points in a second direction perpendicular to the length of the fiber and perpendicular to the first direction. At another point along the length of the fiber, the fast axis points in the first direction and the slow axis points in the second direction. In a fiber with spin, the fast axis traces a generally helical path. The magnitude of the spin can be expressed as the number of turns per unit length of such helix, i.e., the number of times per unit length of fiber that the directions of the fast and slow axes interchange. The direction of the spin corresponds to the direction of the helix traced by the fast axis, either right-handed or left-handed. In a fiber with the appropriate spin, the effects caused by the fast and slow axes are substantially eliminated and all light travels with the same velocity. To provide optimum PMD suppression, it is normally desirable to vary the magnitude and direction of the spin along the length of the fiber.
Various attempts have been made to impart spin to the optical fiber during the production process discussed above. For example, as disclosed in Rashleigh, Navy Technical Disclosure Bulletin, Volume 5, Number 12, Dec. 1980, Navy Tech. Cat. No. 4906, a twisted fiber can be prepared by rotating the preform about its axis while drawing the fiber from the preform. A similar approach, more generally stated as xe2x80x9ccontinuous relative rotation between the preform and the drawn fiberxe2x80x9d is disclosed in International Patent Publication WO 83/00232. As disclosed, for example, in U.S. Pat. No. 4,509,968, the process involving rotation of the preform leads to considerable practical disadvantages. The preform is a massive, soft object which must be maintained at a high temperature. The ""968 patent, therefore, proposes to produce a helical or xe2x80x9cchrialicxe2x80x9d structure in the fiber by feeding the fiber through a set of nips at the cold or downstream end of the fiber drawing process while continually spinning the frame holding the nips. A complex arrangement of a frame and fiber takeup drum is used in this process to transfer the fiber from the spinning nips to the takeup drum.
Hart, Jr., et al. U.S. Pat. Nos. 5,418,881 and 5,298,047 disclose another process for making fibers with spin of alternating clockwise and counterclockwise directions. In this process, the cold end of the fiber passes around a roller while the roller rotates about an axis perpendicular to the longitudinal or upstream-to-downstream direction of the fiber. The roller is periodically moved so that the fiber tends to roll along the surface of the roller, parallel to the axis of rotation of the roller. The fiber periodically slips or jumps along the surface of the roller. Despite these and other efforts in the art, there are still needs for further improvements in processes for imparting a controlled spin to an optical fiber. In particular, there are needs for processes which can provide non-uniform spin, and particularly alternating spins in opposite directions to the fibers in a repeatable, controllable manner. There are corresponding needs for reliable, repeatable apparatus for imparting controlled spins to fibers. In particular, there are needs for methods and apparatus which can impart appreciable spin to a fiber in a repeatable manner during high speed fiber drawing, and which can be used in combination with conventional fiber drawing equipment and processes.
In view of the foregoing, it is an object of the present invention to provide improved methods and apparatus for reducing PMD. More particularly, it is an object of the invention to provide methods and apparatus for imparting spin to an optical fiber in order to reduce PMD.
One aspect of the present invention includes methods of providing spin in an optical fiber. A method according to this aspect of the invention desirably includes the step of drawing the fiber so that the fiber moves, relative to a frame of reference, downstream in a longitudinal direction from a melt zone in which the fiber is soft. The fiber solidifies during this downstream movement. The method further includes the steps of engaging the fiber with surface regions of opposed elements disposed on opposite sides of the solidified fiber downstream from the melt zone and moving these opposed elements so that surface regions of the opposed elements engaged with the fiber move with components of velocity, relative to the frame of reference in the downstream longitudinal direction. The motion is controlled so that during at least part of the drawing step, at least one of the surface regions moves relative to the frame of reference in a lateral direction transverse to the longitudinal direction of the fiber movement and so that the surface regions engaged with the fiber on opposite sides move relative to one another with opposite components of velocity in lateral directions to thereby spin the fiber. Most preferably, the moving step is conducted so that the lateral components of velocity of the surface regions relative to one another are repeatedly reversed, so that the fiber is spun repeatedly in alternating, opposite directions.
According to one aspect of the invention, the step of moving the opposed elements is performed so that the surface region on a first one of the opposed elements moves in a first surface motion direction oblique to the longitudinal direction of the fiber during at least part of the drawing step. Most preferably, the fiber is forcibly engaged with the surfaces of the opposed elements.
According to one aspect of the invention, the step of moving the first element includes the step of moving this first element around a first element axis generally transverse to the longitudinal direction so that the surface region of the first element engaged with the fiber moves perpendicular to this first element axis. For example, the first element may be a roller having a circumferential surface concentric with the first element axis, and the step of moving the first element may include the step of rotating the roller about the first element axis. The first element may also be a belt and the step of moving the first element may include the step of moving the belt around a pulley while the pulley rotates about the first element axis. In either case, the step of moving the first element axis may include the step of rocking the first element, and the first element axis, about a rocking axis transverse to the longitudinal direction of the fiber and also transverse to the aforesaid lateral directions. The rocking axis typically is perpendicular to the first element axis. The surface region of the first element may be a region on the circumferential surface of the roller or on the surface of the belt. When the first element axis rocks about the rocking axis, the direction of motion of this surface portion engaging the fiber (the xe2x80x9cfirst surface motion directionxe2x80x9d) will sweep through a range of angles with respect to the longitudinal direction of the fiber. Desirably, this range extends between first and second equal but opposite extreme angles. The opposed element may be a similar belt or roller and bearing on the opposite side of the fiber, at the same point along the longitudinal direction so that the fiber is squeezed in a nip between the two opposed elements. The second element may be moved around a second element axis, and the second element may be rocked in substantially the same way about a second element rocking axis parallel to or coincident with the rocking axis associated with the first element. Thus, the second surface motion direction swept by the portion of the second element engaging the fiber also sweeps through a range of angles relative to the longitudinal direction. Desirably, the angle between the second surface motion direction and the longitudinal direction is equal but opposite to the angle between the first surface motion direction and the longitudinal direction at all times.
According to a further embodiment of the invention, the second element may include a pair of components, such as a pair of rollers, separated from one another in the longitudinal direction and defining a gap therebetween. The first and second elements are engaged with one another so that the first element is engaged in longitudinal alignment with the gap and so that the first element protrudes into the gap. The fiber is maintained under tension, as by a takeoff stand disposed downstream from the first and second elements and the tension of the fiber forces the fiber against the first and second elements. The method according to this aspect of the invention desirably includes the step of constraining the fiber against motion in the lateral directions at the components or spaced rollers of the second element. Desirably, the rollers include slotted or grooved circumferential surfaces and the fiber is engaged in such surfaces. The second element having the spaced apart components preferably does not move laterally in the fixed or equipment frame of reference. Thus, those portions of the fiber extending upstream and downstream from the first and second elements are not moved laterally during the process. In this arrangement, the surface region of the first element in contact with the fiber moves back and forth in lateral directions relative to the fixed frame of reference. In effect, the fiber rolls around its axis within the grooved surfaces of the second element.
According to yet another embodiment of the invention, the opposed elements may include a pair of rollers having axes transverse to the longitudinal direction of the fiber, or belts extending around pulleys having axes transverse to the longitudinal direction. The step of moving the opposed elements may include the step of moving each such element around its axis while simultaneously translating the elements relative to the fixed frame of reference, preferably in opposite directions.
As the fiber spins around its axis between the opposed elements, the spin is transmitted upstream along the fiber and the fiber is spun within the melt region, thereby imparting a permanent spin to each portion of the fiber. Each portion of the fiber acquires a spin corresponding to the direction of spinning motion during the time such portion of the fiber passed through the melt region and cooled. The fiber may be collected using conventional take up apparatus such as a takeup reel disposed downstream from the opposed elements. Because the spinning motion of the fiber is repeatedly reversed, the fiber is not placed under substantial torsional stress on the takeup reel. Because the fiber moves with controlled rolling motion on the elements of the apparatus, the process is repeatable and predictable. Essentially any amount of spin required for desirable optical properties and essentially any desired pattern of variation in the degree and direction of spin along the length of the fiber can be provided.
Further aspects of the invention provide fiber drawing apparatus. Apparatus according to this aspect of the invention desirably includes a structure defining a melt zone and a solid zone remote from the melt zone as well as means for drawing the fiber along a predetermined path downstream in a longitudinal direction relative to the structure so that the fiber is substantially molten in the melt zone and solidifies during drawing before reaching the solid zone. The apparatus further includes a pair of opposed elements as aforesaid disposed in the solid zone and means for forcibly engaging the opposed elements with the fiber and moving the opposed elements during operation of the fiber drawing means so that surface regions of the opposed elements move relative to one another as discussed above.